When drugs are to be injected into the human body, it is often essential that the dose to be injected is set with a high precision, as a too high or too small dose can have undesirable consequences. At the same time, it should be easy for the patient to set the dose, especially as many patients have poor dexterity. Thus, it is desirable to provide a syringe device wherein the dose is set with a gear mechanism, such that a relative large displacement of a dose setting member results in a relatively small increase or decrease of the set dose.
One such system is known from WO 99/38554, wherein the dose is set by manually rotating an injection button, whereby the button is screwed outwards from the housing. In its movement outwards, the injection button will draw the dose scale drum with it. When the injection button is pressed the movement of this button is transformed into a rotation of the piston rod. The piston rod has an external thread mating an internal thread of a central bore in a wall closing the housing. The threads are designed so that rotation of the piston rod will drive the rod into an ampoule.
Another system is known from WO 2004/028598, which discloses an injection device comprising threaded drive rod in engagement with threads of a rear section of a container holding housing. A lock nut is arranged on the drive rod so as to be slidable but rotationally fixed in relation to the drive rod. A spring is arranged between an end surface of the lock nut and an interior ledge of a dose nut, which comprises a second section having threads on its inner surface in engagement with the threaded drive rod. The force from the spring between the lock nut and the dose nut urges the dose nut away from the lock nut.
Further systems may be seen in US 2002/120235, U.S. Pat. No. 6,413,242, WO 99/38554, WO 94/22507, EP 0 338 806 and U.S. Pat. No. 4,973,318.
In the treatment of certain decreases, e.g. diabetes, patients must perform injections of a drug, such as insulin, on a daily basis. However, such injections can be painful due to the penetration of the needle through the patient's skin and possibly other tissue. Accordingly, syringe devices with thin needles have been developed. However, a small cross-section in the needle, i.e. a narrow passage for the drug, requires a large injecting force. Patients suffering from poor dexterity may not always be able to bring about a large injecting force, or may not feel comfortable providing the required injecting force.
It is thus an object of preferred embodiments of the present invention to provide a syringe device, wherein the injection mechanism does not add to the resistance in the drug ejection system, i.e. in which there is as little resistance in the injection mechanism as possible. Furthermore, it is an object of a preferred embodiment of the present invention to provide a syringe device making it easy for the patient to set a dose with a high precision, even if the patient's dexterity is poor.
A further object of a preferred embodiment of the present invention is to provide an injection assisting system for providing an injection force for assisting an operator in performing an injection.
Additionally, it is an object of a preferred embodiment to provide a syringe device, wherein the piston may be locked for movement in a direction opposite to direction of injection, such that air is not sucked into the drug reservoir by drawing the piston backwardly. Yet another object of a preferred embodiment of the present invention is to provide a syringe device wherein it is possible to decrease a set dose prior to injection, in case a too high dose has been set by mistake.
Furthermore, it is an object of a preferred embodiment of the present invention to provide a syringe device which provides a tactile feedback to the user of the progress of an injection process.